Meeting Abstract
Cortisol plays a critical role in controlling ion regulation in fish, including preparatory adaptations for seawater entry (smolting) that occur just prior to downstream migration and ocean entry of juvenile anadromous salmon. Landlocked Atlantic salmon migrate from streams to lakes, resulting in relaxed selection on traits associated with salinity tolerance but not on other life history changes such as downstream migration and imprinting. In this study anadromous and landlocked strains of Atlantic salmon were reared under identical conditions and examined for differences in seawater performance and its underlying endocrine control during smolt development. Salinity tolerance, survival and growth in the first two weeks of seawater exposure were greater in the anadromous strain. Gill Na+/K+-ATPase (NKA) activity and the abundance of the seawater isoform of gill NKA increased in spring in both strains but were greater in the anadromous strain. Plasma cortisol levels increased in spring in both strains but were 5-fold higher in the anadromous strain in April. Pituitary POMCA1, A2 and B mRNA levels increased in spring but were only slightly different between strains. Hypothalamic mRNA levels of Corticotrophin Releasing Factor (CRF) and Urotensin I (UI) increased in spring and were higher in the anadromous strain. CRF and UI mRNA levels in the preoptic area changed seasonally but were not correlated with circulating cortisol in either strain. The results provide evidence that hypothalamic CRF and UI are involved in increased plasma cortisol during smolting and that the HPI axis has been altered by relaxed selection in landlocked salmon
